MAYMEAD ROBY GREENE ROAD
METEOROLOGICAL STUDY
AND
MODELING ANALYSIS
(FINAL)
July 23, 2001
(as revised December 20, 2001)
Jim Roller
Tom Anderson
Mark Yoder
Jerry Freeman
1
Division of Air Quality
Subject: Meteorology Study – Maymead Roby Greene Road Asphalt
Plant
INTRODUCTION
The purpose of the Maymead Watauga County asphalt plant meteorological study was to
provide site specific meteorology that could be used to conduct refined level modeling
using available EPA approved models such as ISCST3, AERMOD, and CALPUFF.
These models are considered refined level models and provide for more accurate
estimates of ambient concentrations in simple and complex terrain than the screening
level SCREEN3 model. The results of the refined level modeling analyses were also
used to evaluate the relative accuracy and conservatism of the SCREEN3 model in a
complex terrain environment that is similar to many current and proposed asphalt plant
locations in western North Carolina.
METEOROLOGICAL PARAMETERS
The meteorological instrumentation used to collect data at the Maymead – Roby Greene
Rd. site consisted of two 10-meter weather stations located at separate locations; one at
the proposed site referred to as the valley site, the other approximately 400 meters to the
east of the facility and referred to as the hill site. The following meteorological
parameters were collected at each station:
A) ambient temperature at 2 and 10 meters
B) relative humidity
C) solar radiation
D) barometric pressure
E) wind speed and direction at 10m
F) precipitation
The instrumentation was powered by 110-volt current and parameters were sampled
approximately every 2.5 seconds to provide a 1-hour average (1-hour total for
precipitation and solar radiation). Sigma-theta was calculated for wind direction to
provide hourly stability categories.
The data-logger was polled via modem on a regular basis for QA/QC checks and data
archival. DAQ personnel visited the site on a number of occasions to ensure proper
operation and calibration of all systems and to perform equipment repair as needed.
2
GENERAL DISCUSSION
Most regulatory modeling is conducted using the SCREEN3 and ISCST3 gaussian
models to provide conservative estimates of ambient concentrations from one or more
emission sources. These models are capable of providing concentration estimates in all
terrain conditions including the mountainous environment found in western North
Carolina. Modeling in western North Carolina, however, is generally limited to the
SCREEN3 model since site specific or representative meteorology data is usually not
available. Although SCREEN3 using screening meteorology is generally thought to
predict higher impacts than would actually occur, conditions existing at some
mountainous locations (e.g., persistent temperature inversions, stagnant weather
conditions, calm winds, cold air drainage, terrain slope flows, etc,) may degrade model
accuracy and limit the degree of conservatism. ISCST3 and AERMOD using on-site data
will provide a more accurate estimate of ambient concentrations, however, like
SCREEN3, ISCST3 and AERMOD are steady-state models that may not adequately
address all complex terrain concerns.
CALPUFF is a non-steady-state puff dispersion model that can simulate the effects of
time and space varying meteorological conditions on pollutant transport. Three-dimensional
wind fields are created based on multi or single station wind data and the
topography and surface characteristics (e.g., surface roughness, albedo, etc.) within the
modeling domain. CALPUFF can also assess pollutant impacts under stagnant (calm
wind) conditions and through the use of the CALMET model can account for the
kinematic and blocking effects and slope flows of the local terrain.
METEOROLOGICAL DATA
The meteorological data collected from each tower was processed with the upper-air
meteorological data from Blacksburg, Virginia and formatted for use with the ISCST3,
AERMOD, and CALPUFF models. Blacksburg is the nearest upper-air site most
representative of the Roby Greene Road area.
The raw meteorological data sets collected at each tower site represent a full year of
surface data; however, due to the normal time lag of collecting and processing real time
data by the National Climatic Data Center (NCDC), the same year additional
meteorological data (surface and upper-air) required to develop the model meteorological
data sets were not completely available at the time the raw tower data was initially
processed. As a result, the initial model runs conducted for the draft report released in
March 27, 2001, hereafter referred to as draft model runs, were based on meteorological
records for each model as follows:
ISCST3 Nov 1, 1999 – Aug 31, 2000
AERMOD Nov 1, 1999 – Aug 31, 2000
CALPUFF Nov 1, 1999 – Sep 30, 2000
3
The final report issued July 23, 2001, includes additional modeling, hereafter referred to
as final model runs, based on a limited facility operating schedule and the appropriate
subset of the full year (Nov 1, 1999 – Oct 31, 2000) meteorological record for each of the
models runs.
Wind roses have been developed for each of the two meteorological data sets and are
presented in figures 1 and 2. The data represent the full year of data collection –
November 1, 1999 through October 31, 2000. Wind roses are diagrams designed to show
the distribution of wind direction experienced at a given location over a specified time
period (in this case, 1 year). The length of the line is proportional to the frequency of
occurrence of wind from that direction. Wind speed categories are color coded, the
length of which is also proportional to frequency of occurrence. The percentage of calm
winds are calculated and presented in the data summary.
As indicated by the wind roses for each station, winds in the area are predominantly from
the west and northwest with a much smaller frequency of occurrence from the east and
southeast. The “hill” station shows more winds from the northwest as compared to the
“valley” station and is reflective of the specific tower locations. As shown in the
CALMET domain terrain elevation map (figure 3), the valley station is located on the
plant site and would be exposed to winds funneled through the valley in an almost
precisely west-to-east (or vice-versa) direction. The hill station, however, is able to
receive winds more from the northwesterly and southeasterly directions. As reported in
the data summary for each of the wind roses, the percentage of calm winds is in the 40%
to 41% range for each site.
MODEL RUNS
The proposed Maymead Roby Greene Road asphalt plant location is off of Roby Greene
Rd. in Watauga County. The site is situated along the south fork of the New River at an
elevation of approximately 3,100 feet above mean sea level (MSL) and is located in an
east-west oriented river valley bounded to the north and northeast by Chestnut Mountain
and to the northwest and south by several hills. The topography is generally mountainous
with elevations ranging from approximately 3,000 feet above MSL to over 3,500 feet
above MSL within 3 kilometers of the site. The local topography and elevated terrain
surrounding the site is depicted in Figures 4 and 5.
The facility will emit several toxics during the production of asphalt, primarily from the
emissions-controlling baghouse (dryer), truck loadout, and silo operations. The emission
characteristics of each of the emission sources are provided in Table 1. The pollutant
emission rates used in the draft and final report model runs are provided in Table 2 and
are based on the latest DAQ emissions spreadsheet (attachment 1) for hot-mix asphalt
plants (December 2000 final emission factors from Section 11.1 AP-42). A plant
capacity of 150 tons per hour (TPH) was assumed, along with an annual production limit
of 300,000 tons per year (TPY) of asphalt.
4
Table 1.
Point Source Parameters
Source I.D. Baghouse Silo
Location – Easting (m) 443209.71 443231.618
Northing (m) 4010017.632 4010009.5
Source elevation
(m ASL)
946 946
Source height (m) 9.4 13.7
Diameter (m) .86 .1
Exit Temperature (K) 422 422
Exit velocity (m/s) 24.49 .01
Exit flow rate (acm/s) 14.17 7.8E-5
Volume Source Parameters
Source I.D. Loadout
Location – Easting (m) 443231.618
Northing (m) 4010009.503
Source elevation (m ASL) 946
Release height (m) 4.5
Sigma Y (m) .813
Sigma Z (m) .465
Table 2
Toxic Emission Rates*
(lb/hr)
Model Run
Toxic Baghouse Loadout Silo
Draft
Arsenic 1.92E-05 0 0
Benzene 1.34E-02 7.4E-05 1.34E-04
Formaldehyde 4.65E-01 5.49E-04 1.26E-02
Mercury 3.9E-04 0 0
Nickel 9.45E-03 0 0
Final
Arsenic 6.45E-05 0 0
Benzene 4.56E-02 4.56E-04 2.53E-04
Formaldehyde 4.65E-01 5.49E-04 1.26E-02
Mercury 3.9E-04 0 0
Nickel 9.45E-03 0 0
* rates based on 150 TPH plant capacity and 300,0000 TPY production.
5
Draft and Final Model Runs
Using the emissions data shown in Tables 1 and 2, draft and final model runs were
performed to evaluate the off property ambient impacts from the proposed asphalt plant.
The draft model runs were made assuming the asphalt plant would operate at capacity
(150 TPH) for any 1-hour or 24-hour period during the course of the modified year (10
months using ISCST3 and AERMOD, 11 months using CALPUFF) and at a maximum
annual capacity of 300,000 TPY. The final model runs were made making the same
capacity assumptions but further assumed the plant would only operate from 7 am to 7
pm, April 1 through October 31. Although a full 12 months of surface and upper air
meteorological data were available for the final model runs, the limited time frame
modeled represents a more realistic or typical operating scenario for an asphalt plant.
The draft model 1-hour and 24-hour emission rates were derived based on the maximum
hourly production capacity of the asphalt plant (150 TPH). Annual hourly emission rates
were based on total annual facility emissions divided by 8,760 hours which defines the
annual pollutant exposure threshold and acceptable ambient level. The assumption made
in the annual draft model runs was that the additional 1-hour concentrations not modeled
due to the initial lack of additional meteorological data would not result in combined and
averaged concentrations significantly different than those modeled using the ten or eleven
months of available data. A similar assumption was made regarding the 1-hour and 24-
hour draft modeling runs; i.e., the hours not modeled due to the lack of data would not
result in maximum modeled 1-hour or 24-hour impacts significantly different than those
modeled.
The final model 1-hour and 24-hour emission rates, as with the draft model runs, were
derived based on the maximum hourly production capacity of the asphalt plant (150
TPH). Annual hourly emission rates were based on total annual facility emissions
divided by the number of hours the facility would operate in one year. For the final
model runs, the annual hourly emission rate was based on a 12 hour per day, April 1
through October 31, annual operating schedule which equates to 2,568 hours.
The ISCST3, AERMOD, and CALPUFF models were run using the appropriate
processed on-site meteorological data to determine the maximum impact for each
pollutant for each averaging period. The same emission data input stream was used in
each model input file. Details of each model run are provided below.
ISCST3
The ISCST3 (Industrial Source Complex Short-Term) model was designed to
accommodate a variety of emission sources from industrial facilities and is recommended
for use by EPA for rural or urban areas, simple and complex terrain, 1-hour to annual
averaging periods, and continuous pollutant emissions. ISCST3 can account for source
6
separation and building downwash effects at a facility and uses representative or on-site
meteorological data to provide estimates of pollutant concentrations.
Meteorological data requirements for the ISCST3 model include hourly-averaged wind
speed and direction, temperature, stability class, and mixing height. The hourly mixing
heights are calculated following procedures outlined by EPA using data available from
the nearest, most representative National Weather Service station for which upper-air
data is available. Upper-air meteorological parameters are fairly homogeneous over
relatively large areas and the mixing height calculations involve the use of representative
or on-site meteorological data to determine the hourly values.
ISCST3 (00101) was used to evaluate impacts in both simple and complex terrain
surrounding the proposed Maymead Roby Greene facility. The three sources shown in
Table 2 were included in the modeling. Ten months (Nov. 1999 – Aug. 2000) of
meteorological data from the on-site (valley) station with same time frame upper-air data
from Blacksburg, VA were used in the model for the draft model runs. Seven months
(Apr 1 – Oct 31) on-site data were used in the final model runs. Direction-specific
building dimensions, determined using EPA's BPIP program (95086), were used as input
to the model for building wake effect determination. Receptors were placed around the
facility’s property boundary at 25-meter intervals and extended outward to 1.5 kilometers
at 100 meter spacing and from 1.5 km to 3 km at 250 meter spacing. Terrain elevations
were incorporated for each receptor modeled. Maximum impacts for all toxics occurred
in complex terrain. Annual and 24-hour impacts occurred approximately 1 kilometer east
of the facility; 1-hour formaldehyde impacts occurred directly to the south (draft) as well
as to the east (final).
Maximum impacts for each toxic are provided in the modeling summaries given in
attachment 2. The maximum annual impacts in the final model runs were adjusted by the
ratio of the modeled hours to the hours in a full year (5136/8760). Annual pollutant
impacts must be evaluated over a one year (8,760 hours) period The ratio adjustment
was necessary because the ISCST3 model calculated an annual average for the period of
meteorological record (April 1 through October 31 or 5,136 hours) and did not factor or
average in the hours of the year the facility did not operate and in which the hourly
concentrations were zero. Figure 6 shows the impact locations of the maximum ISCST3
modeled concentrations for each pollutant.
AERMOD
AERMOD is a steady-state gaussian model and successor to the ISCST3 model.
AERMOD was designed with the goal of introducing current planetary boundary layer
(PBL) concepts into regulatory dispersion models. Relative to ISC3, AERMOD currently
contains new or improved algorithms for:
1) dispersion in both the convective and stable boundary layers;
2) plume rise and buoyancy;
3) plume penetration into elevated inversions;
7
4) computation of vertical profiles of wind, turbulence, and temperature;
5) the urban boundary layer; and
6) the treatment of receptors on all types of terrain from the surface up to and
above the plume height.
Essentially, AERMOD was developed in an effort to make dispersion modeling more
meteorologically sound. The most notable difference in AERMOD is the way in which
the atmosphere is represented. By creating a temperature and wind speed profile of the
atmosphere, AERMOD graduates beyond the more general temperature and wind speed
categories assigned in ISCST3. AERMOD also considers surface roughness, solar
radiation, surface reflectivity, and surface moisture to more accurately represent the
atmosphere in which a plume is dispersing. With respect to terrain and unlike ISCST3,
AERMOD allows split flow around and over elevated terrain and provides a more
realistic representation of the dispersion of a plume.
One of the major improvements that AERMOD brings to applied dispersion modeling is
its ability to characterize the PBL through both surface and mixed layer scaling.
AERMOD constructs vertical profiles of required meteorological variables based on
measurements and extrapolations of those measurements using similarity (scaling)
relationships. Vertical profiles of wind speed, wind direction, turbulence, temperature,
and temperature gradient are estimated using all available meteorological observations.
AERMOD was designed to run with a minimum of observed meteorological parameters.
As a replacement for the ISCST3 model, AERMOD can operate using data of a type that
is readily available from an NWS station. Although AERMOD can use a representative
and on-site surface data set, AERMOD requires only a single surface measurement of
wind speed (generally at a height of 10m), wind direction and ambient temperature. Like
ISCST3, AERMOD also needs observed cloud cover. However, AERMOD also requires
the full morning upper-air sounding (RAWINSONDE). ISCST3 required only the
morning and afternoon mixing heights derived from the respective morning and
afternoon soundings. In addition, AERMOD needs surface characteristics (surface
roughness, Bowen ratio, and albedo) in order to construct its PBL profiles.
For the Maymead Roby Greene Road site, AERMOD was run using the on-site (valley)
meteorological data, cloud data from Asheville (NWS surface data site), and upper-air
data from Blacksburg, VA (NWS upper-air data site). The period of the data was Nov 1,
1999 to Aug 31, 2000 for the draft model runs and Apr 1, 2000 to Oct 31, 2000 for the
final model runs. The area around the site was designated as a coniferous landmass with
average surface moisture and reflectivity. Annual concentrations were derived based on
the entire available period instead of a full year of met data. The terrain processing
program, AERMAP, was run prior to AERMOD utilizing 7.5 minute USGS DEM data
for the area. All other factors, such as receptors and sources were the same as specified
for the ISCST3 model. Except for formaldehyde (draft), maximum concentrations
occurred in simple terrain in the vicinity of the eastern property line. Formaldehyde
maximum impacts (draft) occurred in the mountains NE of the site.
8
The maximum impacts for each toxic are provided in the modeling summary given in
attachment 2. The AERMOD maximum annual impacts in the final model runs were also
adjusted by the ratio of the modeled hours to the hours in a full year (5136/8760). Like
the ISCST3 model, AERMOD calculated an annual average for the period of
meteorological record (April 1 through October 31 or 5,136 hours) and did not factor or
average in the hours of the year the facility did not operate and in which the hourly
concentrations were zero. Figure 7 shows the impact locations of the maximum
AERMOD modeled concentrations for each pollutant.
CALPUFF
CALPUFF is a non-steady-state dispersion model that can simulate the effects of time-and
space-varying meteorological conditions on pollutant transport. The CALPUFF
modeling system, which includes the CALMET meteorological model, uses three-dimensional
meteorological fields computed by CALMET based on topography, on-site
surface meteorological data, and upper-air sounding data from a nearby NWS station.
Unlike steady-state gaussian models, CALPUFF can assess pollutant impact under
stagnant (calm wind) conditions and through the use of the CALMET model can account
for the kinematic and blocking effects and slope flows of the local terrain.
In order to execute CALPUFF for the time period of concern, CALMET must first be run
in order to obtain a three-dimensional gridded meteorological data field for each hour.
CALMET uses available sources of meteorological and geophysical information to
produce a spatially-varying wind field that is consistent with the local terrain features and
atmospheric stability conditions at the site. A CALMET terrain pre-processor was used to
grid the terrain elevations and land use categories over a 10.2 km x 10.2 km grid
surrounding the proposed Maymead facility. Once the meteorological data fields were
created, CALPUFF was run to calculate pollutant impacts at each receptor in a 10 km x
10 km grid with 100 meter spacing.
The maximum impacts for each toxic are provided in the modeling summary given in
attachment 2. As with ISCST3 and AERMOD and for the same reason, the CALPUFF
maximum annual impacts in the final model runs were adjusted by the ratio of the
modeled hours to the hours in a full year (5136/8760). Figure 8 shows the impact
locations of the maximum CALPUFF modeled concentrations for each pollutant.
MODELING RESULTS
The modeling results for the draft and final refined model runs are presented in
attachment 2. The maximum impact locations are shown in figures 6 through 8. The
noticeable difference in where the maximum impact is predicted to occur between the
refined model results can be attributed to the differences in how each of the models
handles complex terrain and the subtle differences between the closer simple/elevated
terrain impacts and the more distant complex terrain impacts. Of particular note are the
9
CALPUFF maximum impact locations. CALPUFF will track each puff of pollutant
through each grid cell in the modeling domain and react to the differences in meteorology
(e.g., wind direction) in each cell. As suggested by the orientation of the topography and
as shown by the wind rose for the hill tower meteorological data, the prevailing westerly
winds at the emissions site become more northwesterly downwind or east of the facility.
As a result, the puffs of pollutant change course as they move downwind causing
maximum impacts to occur on the hills southeast of the facility as well as the east.
The original and revised SCREEN3 model results using the latest EPA emission factors
are also presented in attachment 2. The first or original SCREEN3 results were
submitted with the initial permit application in May 1998 and reflect emissions based on
existing AP-42 emission factors and DAQ test results. The second SCREEN3 model
results were based on updated EPA asphalt plant emission factors derived in March 2000.
The third or final SCREEN3 model results presented reflect revised EPA emission factors
derived in December 2000. The draft and final refined model runs, including CALPUFF,
show maximum predicted impacts for each pollutant for each averaging period to be less
than the respective pollutant AALs and, with the exception of the draft CALPUFF
formaldehyde impacts, to be less than the SCREEN3 maximum predicted modeling
results.
CONCLUSIONS / RECOMMENDATIONS
The draft and refined model results using site specific meteorology and source emissions
based on a plant capacity of 150 tons per hour and an annual production limit of 300,000
tons of asphalt indicate the proposed Maymead Robey Greene Road asphalt plant would
operate in compliance with the applicable AAL for each of the pollutants evaluated.
Due to the unique meteorological conditions and terrain influences existing in
mountainous terrain such as that in western North Carolina, dispersion modeling becomes
a more difficult process where model deficiencies and uncertainties are more readily
apparent. As a result, caution should be taken in extrapolating the results of this study to
other mountainous locations that may generate different modeling results and
conclusions. However, based on the refined modeling results discussed above and
recognizing that the option of not permitting any industrial facility in western NC due to
these uncertainties or requiring every facility to collect on-site meteorology and conduct
refined level modeling are unlikely scenarios and until additional mountainous location
site specific data can be acquired and used in refined modeling analyses to further
evaluate the effects of complex terrain and plume transport interaction, we recommend
the following:
We believe the meteorology and terrain influences of this site are characteristic of
mountainous terrain in general and as such believe the SCREEN3 model can be used, on
a case-by-case basis and with certain caveats, to evaluate maximum impacts for facility
toxic emissions in mountainous terrain. In addition to determining compliance for the
10
proposed asphalt plant, the collection of meteorological data and subsequent refined
model runs as discussed above were also used to evaluate the relative accuracy and
conservatism of the SCREEN3 model in a complex terrain environment in western North
Carolina. As previously stated, we acknowledge that the specific modeling results and
model comparison percentages presented in this study apply only to the data collection
site. We also acknowledge that SCREEN3 (or for that matter, any screening model) is
not the best model to use to determine pollutant ambient impacts at any given location in
mountainous terrain and that one could expect poor correlation between observed and
predicted values; however, the draft and final refined model results do suggest the
SCREEN3 model may provide, in most cases, conservative estimates of maximum
ambient impacts in such terrain. This is particularly true for annual pollutants where the
technical enhancements incorporated in such models as AERMOD or CALPUFF are
somewhat diminished in the long term averaging process. Also note: the regulatory
modeling process is most concerned with ensuring the maximum modeled ambient
impacts are less than the applicable air quality standards and not with the specific impacts
at any one location. When using SCREEN3, we would recommend a conservative
modeling methodology; e.g., one that would combine maximum impacts from all the
sources evaluated and use the highest 24-hour and annual conversion factors. We would
also recommend that for 1-hour pollutant impacts greater than 50% of the applicable air
quality standard or AAL or for certain terrain influences, e.g., steep terrain contour
gradients, diverging/converging valley orientations, etc., further evaluation should be
conducted to include, as necessary, additional model runs using such models as
CTSCREEN or AERMOD (screening mode) to compare/confirm the SCREEN3 results.
We also recommend that the AQAB continue to evaluate options which would allow the
use of state of the art models such as AERMOD and CALPUFF in western North
Carolina and which would minimize the time consuming and expensive process of
collecting on-site meteorology. Such options may include obtaining/developing a
gridded meteorological database based on MM5 output meteorological data fields
modified by CALPUFF simulations using existing real world meteorological data (e.g.,
Robey Greene, Asheville, etc.). This “representative” meteorological database combined
with site-specific terrain topographic and land-use data could then be used by CALPUFF
or AERMOD to evaluate facility impacts anywhere in western North Carolina. While
technically considered a screening modeling methodology due to lack of site-specific
meteorology, this approach would take advantage of the refined model enhancements and
complex terrain capabilities incorporated in CALPUFF and AERMOD. Such an
approach would significantly reduce complex terrain modeling uncertainties associated
with screening models such as SCREEN3 or CTSCREEN and provide a higher level of
confidence that modeled maximum impacts would be less than the applicable air quality
standards.

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MAYMEAD ROBY GREENE ROAD
METEOROLOGICAL STUDY
AND
MODELING ANALYSIS
(FINAL)
July 23, 2001
(as revised December 20, 2001)
Jim Roller
Tom Anderson
Mark Yoder
Jerry Freeman
1
Division of Air Quality
Subject: Meteorology Study – Maymead Roby Greene Road Asphalt
Plant
INTRODUCTION
The purpose of the Maymead Watauga County asphalt plant meteorological study was to
provide site specific meteorology that could be used to conduct refined level modeling
using available EPA approved models such as ISCST3, AERMOD, and CALPUFF.
These models are considered refined level models and provide for more accurate
estimates of ambient concentrations in simple and complex terrain than the screening
level SCREEN3 model. The results of the refined level modeling analyses were also
used to evaluate the relative accuracy and conservatism of the SCREEN3 model in a
complex terrain environment that is similar to many current and proposed asphalt plant
locations in western North Carolina.
METEOROLOGICAL PARAMETERS
The meteorological instrumentation used to collect data at the Maymead – Roby Greene
Rd. site consisted of two 10-meter weather stations located at separate locations; one at
the proposed site referred to as the valley site, the other approximately 400 meters to the
east of the facility and referred to as the hill site. The following meteorological
parameters were collected at each station:
A) ambient temperature at 2 and 10 meters
B) relative humidity
C) solar radiation
D) barometric pressure
E) wind speed and direction at 10m
F) precipitation
The instrumentation was powered by 110-volt current and parameters were sampled
approximately every 2.5 seconds to provide a 1-hour average (1-hour total for
precipitation and solar radiation). Sigma-theta was calculated for wind direction to
provide hourly stability categories.
The data-logger was polled via modem on a regular basis for QA/QC checks and data
archival. DAQ personnel visited the site on a number of occasions to ensure proper
operation and calibration of all systems and to perform equipment repair as needed.
2
GENERAL DISCUSSION
Most regulatory modeling is conducted using the SCREEN3 and ISCST3 gaussian
models to provide conservative estimates of ambient concentrations from one or more
emission sources. These models are capable of providing concentration estimates in all
terrain conditions including the mountainous environment found in western North
Carolina. Modeling in western North Carolina, however, is generally limited to the
SCREEN3 model since site specific or representative meteorology data is usually not
available. Although SCREEN3 using screening meteorology is generally thought to
predict higher impacts than would actually occur, conditions existing at some
mountainous locations (e.g., persistent temperature inversions, stagnant weather
conditions, calm winds, cold air drainage, terrain slope flows, etc,) may degrade model
accuracy and limit the degree of conservatism. ISCST3 and AERMOD using on-site data
will provide a more accurate estimate of ambient concentrations, however, like
SCREEN3, ISCST3 and AERMOD are steady-state models that may not adequately
address all complex terrain concerns.
CALPUFF is a non-steady-state puff dispersion model that can simulate the effects of
time and space varying meteorological conditions on pollutant transport. Three-dimensional
wind fields are created based on multi or single station wind data and the
topography and surface characteristics (e.g., surface roughness, albedo, etc.) within the
modeling domain. CALPUFF can also assess pollutant impacts under stagnant (calm
wind) conditions and through the use of the CALMET model can account for the
kinematic and blocking effects and slope flows of the local terrain.
METEOROLOGICAL DATA
The meteorological data collected from each tower was processed with the upper-air
meteorological data from Blacksburg, Virginia and formatted for use with the ISCST3,
AERMOD, and CALPUFF models. Blacksburg is the nearest upper-air site most
representative of the Roby Greene Road area.
The raw meteorological data sets collected at each tower site represent a full year of
surface data; however, due to the normal time lag of collecting and processing real time
data by the National Climatic Data Center (NCDC), the same year additional
meteorological data (surface and upper-air) required to develop the model meteorological
data sets were not completely available at the time the raw tower data was initially
processed. As a result, the initial model runs conducted for the draft report released in
March 27, 2001, hereafter referred to as draft model runs, were based on meteorological
records for each model as follows:
ISCST3 Nov 1, 1999 – Aug 31, 2000
AERMOD Nov 1, 1999 – Aug 31, 2000
CALPUFF Nov 1, 1999 – Sep 30, 2000
3
The final report issued July 23, 2001, includes additional modeling, hereafter referred to
as final model runs, based on a limited facility operating schedule and the appropriate
subset of the full year (Nov 1, 1999 – Oct 31, 2000) meteorological record for each of the
models runs.
Wind roses have been developed for each of the two meteorological data sets and are
presented in figures 1 and 2. The data represent the full year of data collection –
November 1, 1999 through October 31, 2000. Wind roses are diagrams designed to show
the distribution of wind direction experienced at a given location over a specified time
period (in this case, 1 year). The length of the line is proportional to the frequency of
occurrence of wind from that direction. Wind speed categories are color coded, the
length of which is also proportional to frequency of occurrence. The percentage of calm
winds are calculated and presented in the data summary.
As indicated by the wind roses for each station, winds in the area are predominantly from
the west and northwest with a much smaller frequency of occurrence from the east and
southeast. The “hill” station shows more winds from the northwest as compared to the
“valley” station and is reflective of the specific tower locations. As shown in the
CALMET domain terrain elevation map (figure 3), the valley station is located on the
plant site and would be exposed to winds funneled through the valley in an almost
precisely west-to-east (or vice-versa) direction. The hill station, however, is able to
receive winds more from the northwesterly and southeasterly directions. As reported in
the data summary for each of the wind roses, the percentage of calm winds is in the 40%
to 41% range for each site.
MODEL RUNS
The proposed Maymead Roby Greene Road asphalt plant location is off of Roby Greene
Rd. in Watauga County. The site is situated along the south fork of the New River at an
elevation of approximately 3,100 feet above mean sea level (MSL) and is located in an
east-west oriented river valley bounded to the north and northeast by Chestnut Mountain
and to the northwest and south by several hills. The topography is generally mountainous
with elevations ranging from approximately 3,000 feet above MSL to over 3,500 feet
above MSL within 3 kilometers of the site. The local topography and elevated terrain
surrounding the site is depicted in Figures 4 and 5.
The facility will emit several toxics during the production of asphalt, primarily from the
emissions-controlling baghouse (dryer), truck loadout, and silo operations. The emission
characteristics of each of the emission sources are provided in Table 1. The pollutant
emission rates used in the draft and final report model runs are provided in Table 2 and
are based on the latest DAQ emissions spreadsheet (attachment 1) for hot-mix asphalt
plants (December 2000 final emission factors from Section 11.1 AP-42). A plant
capacity of 150 tons per hour (TPH) was assumed, along with an annual production limit
of 300,000 tons per year (TPY) of asphalt.
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Table 1.
Point Source Parameters
Source I.D. Baghouse Silo
Location – Easting (m) 443209.71 443231.618
Northing (m) 4010017.632 4010009.5
Source elevation
(m ASL)
946 946
Source height (m) 9.4 13.7
Diameter (m) .86 .1
Exit Temperature (K) 422 422
Exit velocity (m/s) 24.49 .01
Exit flow rate (acm/s) 14.17 7.8E-5
Volume Source Parameters
Source I.D. Loadout
Location – Easting (m) 443231.618
Northing (m) 4010009.503
Source elevation (m ASL) 946
Release height (m) 4.5
Sigma Y (m) .813
Sigma Z (m) .465
Table 2
Toxic Emission Rates*
(lb/hr)
Model Run
Toxic Baghouse Loadout Silo
Draft
Arsenic 1.92E-05 0 0
Benzene 1.34E-02 7.4E-05 1.34E-04
Formaldehyde 4.65E-01 5.49E-04 1.26E-02
Mercury 3.9E-04 0 0
Nickel 9.45E-03 0 0
Final
Arsenic 6.45E-05 0 0
Benzene 4.56E-02 4.56E-04 2.53E-04
Formaldehyde 4.65E-01 5.49E-04 1.26E-02
Mercury 3.9E-04 0 0
Nickel 9.45E-03 0 0
* rates based on 150 TPH plant capacity and 300,0000 TPY production.
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Draft and Final Model Runs
Using the emissions data shown in Tables 1 and 2, draft and final model runs were
performed to evaluate the off property ambient impacts from the proposed asphalt plant.
The draft model runs were made assuming the asphalt plant would operate at capacity
(150 TPH) for any 1-hour or 24-hour period during the course of the modified year (10
months using ISCST3 and AERMOD, 11 months using CALPUFF) and at a maximum
annual capacity of 300,000 TPY. The final model runs were made making the same
capacity assumptions but further assumed the plant would only operate from 7 am to 7
pm, April 1 through October 31. Although a full 12 months of surface and upper air
meteorological data were available for the final model runs, the limited time frame
modeled represents a more realistic or typical operating scenario for an asphalt plant.
The draft model 1-hour and 24-hour emission rates were derived based on the maximum
hourly production capacity of the asphalt plant (150 TPH). Annual hourly emission rates
were based on total annual facility emissions divided by 8,760 hours which defines the
annual pollutant exposure threshold and acceptable ambient level. The assumption made
in the annual draft model runs was that the additional 1-hour concentrations not modeled
due to the initial lack of additional meteorological data would not result in combined and
averaged concentrations significantly different than those modeled using the ten or eleven
months of available data. A similar assumption was made regarding the 1-hour and 24-
hour draft modeling runs; i.e., the hours not modeled due to the lack of data would not
result in maximum modeled 1-hour or 24-hour impacts significantly different than those
modeled.
The final model 1-hour and 24-hour emission rates, as with the draft model runs, were
derived based on the maximum hourly production capacity of the asphalt plant (150
TPH). Annual hourly emission rates were based on total annual facility emissions
divided by the number of hours the facility would operate in one year. For the final
model runs, the annual hourly emission rate was based on a 12 hour per day, April 1
through October 31, annual operating schedule which equates to 2,568 hours.
The ISCST3, AERMOD, and CALPUFF models were run using the appropriate
processed on-site meteorological data to determine the maximum impact for each
pollutant for each averaging period. The same emission data input stream was used in
each model input file. Details of each model run are provided below.
ISCST3
The ISCST3 (Industrial Source Complex Short-Term) model was designed to
accommodate a variety of emission sources from industrial facilities and is recommended
for use by EPA for rural or urban areas, simple and complex terrain, 1-hour to annual
averaging periods, and continuous pollutant emissions. ISCST3 can account for source
6
separation and building downwash effects at a facility and uses representative or on-site
meteorological data to provide estimates of pollutant concentrations.
Meteorological data requirements for the ISCST3 model include hourly-averaged wind
speed and direction, temperature, stability class, and mixing height. The hourly mixing
heights are calculated following procedures outlined by EPA using data available from
the nearest, most representative National Weather Service station for which upper-air
data is available. Upper-air meteorological parameters are fairly homogeneous over
relatively large areas and the mixing height calculations involve the use of representative
or on-site meteorological data to determine the hourly values.
ISCST3 (00101) was used to evaluate impacts in both simple and complex terrain
surrounding the proposed Maymead Roby Greene facility. The three sources shown in
Table 2 were included in the modeling. Ten months (Nov. 1999 – Aug. 2000) of
meteorological data from the on-site (valley) station with same time frame upper-air data
from Blacksburg, VA were used in the model for the draft model runs. Seven months
(Apr 1 – Oct 31) on-site data were used in the final model runs. Direction-specific
building dimensions, determined using EPA's BPIP program (95086), were used as input
to the model for building wake effect determination. Receptors were placed around the
facility’s property boundary at 25-meter intervals and extended outward to 1.5 kilometers
at 100 meter spacing and from 1.5 km to 3 km at 250 meter spacing. Terrain elevations
were incorporated for each receptor modeled. Maximum impacts for all toxics occurred
in complex terrain. Annual and 24-hour impacts occurred approximately 1 kilometer east
of the facility; 1-hour formaldehyde impacts occurred directly to the south (draft) as well
as to the east (final).
Maximum impacts for each toxic are provided in the modeling summaries given in
attachment 2. The maximum annual impacts in the final model runs were adjusted by the
ratio of the modeled hours to the hours in a full year (5136/8760). Annual pollutant
impacts must be evaluated over a one year (8,760 hours) period The ratio adjustment
was necessary because the ISCST3 model calculated an annual average for the period of
meteorological record (April 1 through October 31 or 5,136 hours) and did not factor or
average in the hours of the year the facility did not operate and in which the hourly
concentrations were zero. Figure 6 shows the impact locations of the maximum ISCST3
modeled concentrations for each pollutant.
AERMOD
AERMOD is a steady-state gaussian model and successor to the ISCST3 model.
AERMOD was designed with the goal of introducing current planetary boundary layer
(PBL) concepts into regulatory dispersion models. Relative to ISC3, AERMOD currently
contains new or improved algorithms for:
1) dispersion in both the convective and stable boundary layers;
2) plume rise and buoyancy;
3) plume penetration into elevated inversions;
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4) computation of vertical profiles of wind, turbulence, and temperature;
5) the urban boundary layer; and
6) the treatment of receptors on all types of terrain from the surface up to and
above the plume height.
Essentially, AERMOD was developed in an effort to make dispersion modeling more
meteorologically sound. The most notable difference in AERMOD is the way in which
the atmosphere is represented. By creating a temperature and wind speed profile of the
atmosphere, AERMOD graduates beyond the more general temperature and wind speed
categories assigned in ISCST3. AERMOD also considers surface roughness, solar
radiation, surface reflectivity, and surface moisture to more accurately represent the
atmosphere in which a plume is dispersing. With respect to terrain and unlike ISCST3,
AERMOD allows split flow around and over elevated terrain and provides a more
realistic representation of the dispersion of a plume.
One of the major improvements that AERMOD brings to applied dispersion modeling is
its ability to characterize the PBL through both surface and mixed layer scaling.
AERMOD constructs vertical profiles of required meteorological variables based on
measurements and extrapolations of those measurements using similarity (scaling)
relationships. Vertical profiles of wind speed, wind direction, turbulence, temperature,
and temperature gradient are estimated using all available meteorological observations.
AERMOD was designed to run with a minimum of observed meteorological parameters.
As a replacement for the ISCST3 model, AERMOD can operate using data of a type that
is readily available from an NWS station. Although AERMOD can use a representative
and on-site surface data set, AERMOD requires only a single surface measurement of
wind speed (generally at a height of 10m), wind direction and ambient temperature. Like
ISCST3, AERMOD also needs observed cloud cover. However, AERMOD also requires
the full morning upper-air sounding (RAWINSONDE). ISCST3 required only the
morning and afternoon mixing heights derived from the respective morning and
afternoon soundings. In addition, AERMOD needs surface characteristics (surface
roughness, Bowen ratio, and albedo) in order to construct its PBL profiles.
For the Maymead Roby Greene Road site, AERMOD was run using the on-site (valley)
meteorological data, cloud data from Asheville (NWS surface data site), and upper-air
data from Blacksburg, VA (NWS upper-air data site). The period of the data was Nov 1,
1999 to Aug 31, 2000 for the draft model runs and Apr 1, 2000 to Oct 31, 2000 for the
final model runs. The area around the site was designated as a coniferous landmass with
average surface moisture and reflectivity. Annual concentrations were derived based on
the entire available period instead of a full year of met data. The terrain processing
program, AERMAP, was run prior to AERMOD utilizing 7.5 minute USGS DEM data
for the area. All other factors, such as receptors and sources were the same as specified
for the ISCST3 model. Except for formaldehyde (draft), maximum concentrations
occurred in simple terrain in the vicinity of the eastern property line. Formaldehyde
maximum impacts (draft) occurred in the mountains NE of the site.
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The maximum impacts for each toxic are provided in the modeling summary given in
attachment 2. The AERMOD maximum annual impacts in the final model runs were also
adjusted by the ratio of the modeled hours to the hours in a full year (5136/8760). Like
the ISCST3 model, AERMOD calculated an annual average for the period of
meteorological record (April 1 through October 31 or 5,136 hours) and did not factor or
average in the hours of the year the facility did not operate and in which the hourly
concentrations were zero. Figure 7 shows the impact locations of the maximum
AERMOD modeled concentrations for each pollutant.
CALPUFF
CALPUFF is a non-steady-state dispersion model that can simulate the effects of time-and
space-varying meteorological conditions on pollutant transport. The CALPUFF
modeling system, which includes the CALMET meteorological model, uses three-dimensional
meteorological fields computed by CALMET based on topography, on-site
surface meteorological data, and upper-air sounding data from a nearby NWS station.
Unlike steady-state gaussian models, CALPUFF can assess pollutant impact under
stagnant (calm wind) conditions and through the use of the CALMET model can account
for the kinematic and blocking effects and slope flows of the local terrain.
In order to execute CALPUFF for the time period of concern, CALMET must first be run
in order to obtain a three-dimensional gridded meteorological data field for each hour.
CALMET uses available sources of meteorological and geophysical information to
produce a spatially-varying wind field that is consistent with the local terrain features and
atmospheric stability conditions at the site. A CALMET terrain pre-processor was used to
grid the terrain elevations and land use categories over a 10.2 km x 10.2 km grid
surrounding the proposed Maymead facility. Once the meteorological data fields were
created, CALPUFF was run to calculate pollutant impacts at each receptor in a 10 km x
10 km grid with 100 meter spacing.
The maximum impacts for each toxic are provided in the modeling summary given in
attachment 2. As with ISCST3 and AERMOD and for the same reason, the CALPUFF
maximum annual impacts in the final model runs were adjusted by the ratio of the
modeled hours to the hours in a full year (5136/8760). Figure 8 shows the impact
locations of the maximum CALPUFF modeled concentrations for each pollutant.
MODELING RESULTS
The modeling results for the draft and final refined model runs are presented in
attachment 2. The maximum impact locations are shown in figures 6 through 8. The
noticeable difference in where the maximum impact is predicted to occur between the
refined model results can be attributed to the differences in how each of the models
handles complex terrain and the subtle differences between the closer simple/elevated
terrain impacts and the more distant complex terrain impacts. Of particular note are the
9
CALPUFF maximum impact locations. CALPUFF will track each puff of pollutant
through each grid cell in the modeling domain and react to the differences in meteorology
(e.g., wind direction) in each cell. As suggested by the orientation of the topography and
as shown by the wind rose for the hill tower meteorological data, the prevailing westerly
winds at the emissions site become more northwesterly downwind or east of the facility.
As a result, the puffs of pollutant change course as they move downwind causing
maximum impacts to occur on the hills southeast of the facility as well as the east.
The original and revised SCREEN3 model results using the latest EPA emission factors
are also presented in attachment 2. The first or original SCREEN3 results were
submitted with the initial permit application in May 1998 and reflect emissions based on
existing AP-42 emission factors and DAQ test results. The second SCREEN3 model
results were based on updated EPA asphalt plant emission factors derived in March 2000.
The third or final SCREEN3 model results presented reflect revised EPA emission factors
derived in December 2000. The draft and final refined model runs, including CALPUFF,
show maximum predicted impacts for each pollutant for each averaging period to be less
than the respective pollutant AALs and, with the exception of the draft CALPUFF
formaldehyde impacts, to be less than the SCREEN3 maximum predicted modeling
results.
CONCLUSIONS / RECOMMENDATIONS
The draft and refined model results using site specific meteorology and source emissions
based on a plant capacity of 150 tons per hour and an annual production limit of 300,000
tons of asphalt indicate the proposed Maymead Robey Greene Road asphalt plant would
operate in compliance with the applicable AAL for each of the pollutants evaluated.
Due to the unique meteorological conditions and terrain influences existing in
mountainous terrain such as that in western North Carolina, dispersion modeling becomes
a more difficult process where model deficiencies and uncertainties are more readily
apparent. As a result, caution should be taken in extrapolating the results of this study to
other mountainous locations that may generate different modeling results and
conclusions. However, based on the refined modeling results discussed above and
recognizing that the option of not permitting any industrial facility in western NC due to
these uncertainties or requiring every facility to collect on-site meteorology and conduct
refined level modeling are unlikely scenarios and until additional mountainous location
site specific data can be acquired and used in refined modeling analyses to further
evaluate the effects of complex terrain and plume transport interaction, we recommend
the following:
We believe the meteorology and terrain influences of this site are characteristic of
mountainous terrain in general and as such believe the SCREEN3 model can be used, on
a case-by-case basis and with certain caveats, to evaluate maximum impacts for facility
toxic emissions in mountainous terrain. In addition to determining compliance for the
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proposed asphalt plant, the collection of meteorological data and subsequent refined
model runs as discussed above were also used to evaluate the relative accuracy and
conservatism of the SCREEN3 model in a complex terrain environment in western North
Carolina. As previously stated, we acknowledge that the specific modeling results and
model comparison percentages presented in this study apply only to the data collection
site. We also acknowledge that SCREEN3 (or for that matter, any screening model) is
not the best model to use to determine pollutant ambient impacts at any given location in
mountainous terrain and that one could expect poor correlation between observed and
predicted values; however, the draft and final refined model results do suggest the
SCREEN3 model may provide, in most cases, conservative estimates of maximum
ambient impacts in such terrain. This is particularly true for annual pollutants where the
technical enhancements incorporated in such models as AERMOD or CALPUFF are
somewhat diminished in the long term averaging process. Also note: the regulatory
modeling process is most concerned with ensuring the maximum modeled ambient
impacts are less than the applicable air quality standards and not with the specific impacts
at any one location. When using SCREEN3, we would recommend a conservative
modeling methodology; e.g., one that would combine maximum impacts from all the
sources evaluated and use the highest 24-hour and annual conversion factors. We would
also recommend that for 1-hour pollutant impacts greater than 50% of the applicable air
quality standard or AAL or for certain terrain influences, e.g., steep terrain contour
gradients, diverging/converging valley orientations, etc., further evaluation should be
conducted to include, as necessary, additional model runs using such models as
CTSCREEN or AERMOD (screening mode) to compare/confirm the SCREEN3 results.
We also recommend that the AQAB continue to evaluate options which would allow the
use of state of the art models such as AERMOD and CALPUFF in western North
Carolina and which would minimize the time consuming and expensive process of
collecting on-site meteorology. Such options may include obtaining/developing a
gridded meteorological database based on MM5 output meteorological data fields
modified by CALPUFF simulations using existing real world meteorological data (e.g.,
Robey Greene, Asheville, etc.). This “representative” meteorological database combined
with site-specific terrain topographic and land-use data could then be used by CALPUFF
or AERMOD to evaluate facility impacts anywhere in western North Carolina. While
technically considered a screening modeling methodology due to lack of site-specific
meteorology, this approach would take advantage of the refined model enhancements and
complex terrain capabilities incorporated in CALPUFF and AERMOD. Such an
approach would significantly reduce complex terrain modeling uncertainties associated
with screening models such as SCREEN3 or CTSCREEN and provide a higher level of
confidence that modeled maximum impacts would be less than the applicable air quality
standards.